The E787 and E949 experiments at the Brookhaven National Laboratory
studied the \(K^+\rightarrow \pi ^+\nu \bar{\nu }\)  decay using a kaon decay-at-rest technique and
measured BR=\((17.3^{+11.5}_{-10.5})\times 10^{-11}\) 
[1]}. More recently, the NA62 experiment at the CERN Super
Proton Synchrotron (SPS) has measured the BR more precisely to be
\((4.8^{+7.2}_{-4.8})\times 10^{-11}\)  using a decay-in-flight technique
and data recorded in 2016 [2]} and 2017 [3]}.
